Modulation system



July 9, 1940. J 2,207,498

MODULATION SYSTEM Filed June 16, 1938 SUPPLEMENTARY MODULATED POWER 40 5sup smas 3 ILL 1 fico 2 4E RE MODULATING MODULATl/VG .1 Q 2 ma POTENMLSPOTENTIAL MODULATOR FREQUENCY AMPLIFIER .souRcs "C I i Q Q g m w fi Q 3w PERCE/VMGE} MODULATION J Egg/$ BY CARR/ER CONDITION I A TTORNEY.

Patented July 9, 1940 UNITE STATES PATENT OFFICE MODULATION SYSTEM ofDelaware Application June 16,1938, Serial No. 213,984 In Great BritainJune 16, 1937 12 Claims.

This invention relates to radio and other modulated carrier wavetransmitters and has for its object to provide an improved transmitterwhich shall have a high operating efficiency and shall nevertheless notinvolve the use of the large and expensive iron-cored components,notably large and expensive iron-cored transformers, such as arerequired with present day known transmitters of comparable efficiency.

According to this invention a modulated carrier wave transmittercomprises a main tube stage which feeds a load, for example, an aerial,and which receives a high frequency drive, the said stage being operatedso as a so-called Class C amplifier; an audio frequency choke in thehigh tension feed circuit of said main tube stage; a modulator tubestage which is associated with the main tube stage so as to apply anodemodulation thereto by the choke method, the said modulator tube stagebeing so biased as to effect modulation substantially only over thenegative half cycles of the envelope; and a supplementary modulatedpower supply stage which receives a modulated high frequency drive inphase with the drive on the main tube stage and is connected to supplymodulated power to the load, said supplementary tube stage being sobiased as to take increased feed and thus deliver power to the loadduring positive half cycles of. modulation, and the Whole arrangementbeing such that said supplementary tube stage by delivering power to theload, causes the main tube stage to draw on energy stored in said audiofrequency choke so that said main tube stage delivers increased power tothe load in accordance with the power delivered by the supplementarytube stage. The drive to the main tube stage may be a pure carrier driveor it may be a modulated carrier drive. In the latter case the drive tothe said main tube stage and that to the supplementary stage may betaken from the same source. The power supplied by the supplementarystage to the load may be supplied thereto via the anode circuit of themain tube stage.

The invention is illustrated in the accompanying drawing in which Figure1 shows diagrammatically and by way of example, one circuit arrangementin accordance with the invention and Figure 2 is an explanatorygraphical figure.

Referring to Figure 1 the anode i of the tube 2 in the main tube stageis connected to the positive terminal 3 of a high tension supply source(not shown) through a radio frequency choke 3 and an audio frequencychoke 5 in series.

The grid 6 of the main tube stage is connected;

to the cathode 1 thereof through a grid leak resistance 8 the cathode ibeing connected to the negative terminal 9 of the high tension supply.The anode circuit of the main tube stage includes a coupling condenser lwhich is connected on one side to the anode l and on the other to oneend of a parallel tuned circuit l I, I2, whose other end is connected tothe negative terminal 9 of the high tension supply. The load circuit isfed from this parallel tuned circuit and may be regarded as a resistancein shunt thereacross. In Fig. l, the load is represented by a resistanceI3 thus connected. High frequency unmodulated carrier drive from anysuitable source i is supplied to the control grid 6 of the tube 2through a condenser IB and a carrier drive from the. same source is alsosupplied to a modulated amplifier ll of any convenientknown type whosemodulated output is fed byway of a condenser it to the control grid 19of the tube 20 of a supplementary modulated power supply stage the anode2| of which is connected to the high tension positive terminal 3 througha radio frequency choke 22. The cathode 23 of, the tube 20 is connectedto the high tension negative terminal 9 and the control grid [9 of thistube is connected to said negative terminal 9 through a radio fre-vquency choke 24 in series with a negative bias source 25 of such valueas to bias the tube 29 to twice the cut-off value. The anode 2! of thesupplementary tube 20 is connected through a condenser 26 to a point 21on the inductance II in the tuned circuit l I, I2, this point 21 beingabout halfway down on the inductance. Speech or other modulating signalsare supplied overlead 28 to a modulation potential amplifier 29 of anysuitable known type, output from which is taken to the modulatedamplifier l1 and also through a condenser 30 to the control grid 3! ofthe tube 32 of a modulator stage whose anode 33 is connected through theaudio frequency choke 5 to the high tension positive terminal 3. Thecontrol grid 3| of the modulator-tube 32 is connected through aresistance 34 and a negative bias source 35 to the cathode 36 and thenegative terminal 9 of the high tension supply. The bias of themodulator tube 32 is set approximately to cut-oil.

With this arrangement the main tube 2 receives constant drive andoperates as a high efficiency Class C amplifier the ratio of peak radiofrequency voltage to high tension voltage on its anode I being about0.9. The drive and bias upon the supplementary tube 20 are such that asmall anode feed flows to this stage in the zero modulation or carriercondition and since the tap at 21, from the anode of this stage upon thetuned circuit ll, l2, in the anode circuit of the main tube stage isabout half way down on the inductance H the ratio of radio frequencypeak voltage to high tension voltage on the anode 2! of thesupplementary tubeZD will be about 0.45 in the carrier condition. Duringpositive half cycles of modulation the supplementary stage is drivenharder" and accordingly takes increased feed and delivers increasedpower to the load and accordingly the ratio of radio frequency voltageto high tension voltage on the anode l of the main tube stage isincreased and the said main tube stage tends to take decreased feed.This, however, is resisted by the action of the audio frequency choke 5which automatically raises the potential at its end which is nearer theanode l of the main tube stage so as to maintain the feed constantuntil, with positive peaks of modulation, the potential at the said endis approximately twice the high tension voltage. In this condition themain tube stage is delivering very nearly twice carrier power into theload and a similar amount apprommately to twice carrier power issupplied by the supplementary stage. The choice of the ratio of 0.45already referred to, as occurring in the carrer condition, allows alimiting ratio value of 0.9 to be reached in the 100% modulationcondition. The results obtained are graphically represented in Figure 2in which curve AC2 is a curve of anode current in the tube 2; curve A020is a curve of anode current in the tube 2!]; curve AC32 is a curve ofanode current in the tube 32; curve P2 is a curve of power delivered bytube 2 to the load; curve P20 is a curve of power delivered by tube 20to the load; andTP is a curve of total power in the load; all the curvesbeing drawn against percentage modulation as abscissae. It will beobserved that AC=2AB=4ED. As will'be seen the anode current of the tube20 rises from practically zero at carrier level (point E) to a maximumat the positive peaks of 100% modulation; while for the negative halfcycles the anode current of the modulator tube 32 rises approximatelyrectilinearly from almost zero at carrier level (point E) to a maximumat the negative peaks of 100%. During modulation over the range from100% negative to 100% positive in the modulation cycle the anode currentof the main tube 2 rises approximately rectilinearly from zero (at thenegative end of this range) to a maximum at the carrier condition,remaining substantially at that maximum over the positive half of thesaid range. For any depth of modulation the potential at that end of theaudio frequency choke 5 nearer the anode I of the main tube 2, variessymmetrically about the high tension supply voltage and the feed to themain tube 2 and to the modulator tube 32 remains very nearly constantover each modulation cycle. The average resistance provided by these twostages is, therefore, practically unaffected by modulation and thecarrier level will not materially vary during changing conditions ofmodulation. The power drawn from the audio frequency choke 5 duringpositive half cycles of modulation is applied thereto during negativehalf cycles.

As already stated the main tube 2 may receive, if desired, a modulateddrive instead of a pure carrier drive. To supply this modulated drivethe switch S is moved from the contact CD by which carrier drive issupplied to the grid 6 of tube 2 to the contact MOD so that modulatedcarrier drive is supplied to the control grid 6.

Further, the anode 33 of the modulator tube 32 may be taken to a tap 40on the audio choke 5 a short distance from that end thereof which isnearer the anode I of the main tube 2 and this tap may be so chosen asto permit 100% modulation without requiring that the anode 33 shouldfall to zero potential on the negative peaks. To connect the anode 33 oftube 32 to the tap 48 the switch SI is moved from the contact M to thecontact 42.

Obviously the various stages need not be single tube stages as shown forany or all of them may include a plurality of tubes in parallel or othersuitable connection as may be desired.

It is worthy of note that transmitters in accordance with this inventioncan be designed to give an efificiency under all conditions ofmodulation which is as good as and indeed rather better than thatobtained with the well known socalled Class B modulator system butnevertheless does not involve the use of the expensive and largepush-pull speech transformer required by the said known Class Bmodulator system. Further, although the main tube stage supplies almostall the carrier power it need be rated at only half the rating of themain amplifier stage in a corresponding known Class B modulator systembecause in the present case there is no increase in the mean or peakanode current during positive peaks of modulation so that the main tubestage can be designed to approach saturation even in the carriercondition. Broadly speaking it may be said that the main tube stage andthe supplementary stage together correspond approximately to the mainamplifier stage of a known Class B modulator system while the modulatorstage corresponds to one half of the push-pull modulator in the saidknown system so that the economy in apparatus as compared to the saidknown system is equivalent to about one half of the modulator of thesaid known system.

I claim:

1. A modulated carrier wave transmitter comprising a main tube stagehaving in its anode circuit a tuned circuit from which a load is fed,the anode of said stage being connected to a source of anode potentialthrough a radio frequency choke and an audio frequency choke and saidstage being operated as a Class C amplifier, means for supplying acarrier frequency drive to the grid of said main tube stage, asupplementary tube stage which is biased to approximately twice cut-offvalue and has its anode connected through a radio frequency choke to thesource of anode potential, and coupled to a point about half way down onsaid tuned circuit, means for supplying a modulated carrier drive to thegrid of said supplementary stage, and a modulator tube stage biasedapproximately to cut-off and connected to receive a modulating signalinput on its grid, the anode of the modulator stage being connected tothe source of anode potential through said audio frequency choke.

2. A modification of a trasmitter as claimed in claim 1 wherein theanode of the modulator stage is connected to the source of anodepotential through only part of said choke.

3. A modification of a transmitter as claimed in claim 1 wherein themain tube stage receives a modulated drive instead of a pure carrierdrive.

4. In a modulation system, a main amplifier tube having input and outputelectrodes, an auxiliary amplifier tube having input and outputelectrodes, a source of modulating potentials and a source of waveenergy to be modulated, a

load circuit coupled to the output electrodes of both of said tubes,means supplying direct current voltages to the output electrodes of bothof said tubes, means interconnecting said sources and said tubes, meansfor biasing the electrodes of said auxiliary amplifier tube to such apotential difference that said auxiliary amplifier tube supplies energyto said load circuit only in the presence of wave amplitudes of carrieramplitude or greater whereby the effective impedance presented by theload circuit to said main tube increases on upward swings of themodulating potential, means for holding the direct current supply tosaid main tube substantially constant on said upward swings, and meansfor decreasing the plate voltage of said main tube on downward swings ofsaid modulating potentials.

5. In a modulation system, a load impedance, a source of carrier waveenergy to be modulated, a source of modulating potentials, a mainelectron discharge tube having input electrodes and having outputelectrodes coupled to said load impedance, means for impressing highfrequency wave energy on said input electrodes, an auxiliary amplifiertube having input electrodes and having output electrodes coupled tosaid load impedance, means for supplying a substantially constant amountof direct current to the output electrodes of said main tube, meansconnected with said sources and the input electrodes of said auxiliarytube for supplying modulated wave energy to said input electrodes ofsaid auxiliary tube, means for biasing said auxiliary tube to passsubstantially zero current in the presence of wave energy or carrierwave amplitude or less, a modulator tube having output electrodesconnected with said direct current supply means and having inputelectrodes coupled to said source of modulating potentials, and meansfor biasing electrodes of said modulator tube to pass current only onthe negative portion of the modulation potential cycle.

6. A modulated carrier wave transmitter comprising a main electrondischarge tube amplifier stage, means for exciting said tube by avoltage of carrier wave frequency, a load circuit coupled to said tubeto be excited thereby, means for operating said tube as a Class Camplifier, and audio frequency choke in a direct current feed circuitfor said main tube, a modulator tube connected to said direct currentfeed circuit to apply anode modulation to said main tube by the chokemethod, means for biasing the said modulator tube and controlling thesame to effect modulation substantially only over the negative halfcycles of the modulation envelope, a supplementary modulated poweramplifier tube excited by a modulated voltage of carrier wave frequencyin phase with the exciting voltage on the main tube, means connectingsaid last amplifier tube to said load circuit to supply modulated powerto the load, and means for biasing said supplementary tube to takeincreased feed and thus deliver power to the load substantially duringpositive half cycles of modulation only whereby said supplementary tubestage by delivering power to the load causes the main tube stage to drawon energy stored. in said audio frequency choke so that said main tubestage delivers increased power to the load in accordance with the powerdelivered by the supplementary tube stage. 7. A transmitter as recitedin claim 6 wherein the drive to the main tube stage is a pure carrierdrive.

8, A transmitter as recited in claim 6 wherein the drive to the maintube stage is a moduin the main tube has an anode circuit coupling it tothe load circuit and wherein the power supplied by the supplementarystage to the load is supplied thereto via the said anode circuit of themain tube stage.

11. In a modulation system, a main amplifier tube having inputelectrodes and having output electrodes connected with a substantiallyconstant direct current source, a source of modulating potentials and asource of wave energy to be modulated, means for impressing wave energyto be modulated on said input electrodes, a load circuit coupled to theoutput electrode of said amplifier tube, an auxiliary tube having inputelectrodes excited by wave energy from said source of wave energy andhaving output electrodes coupled to said load circuit, means forincreasing the output of said auxiliary tube on the upward swings ofsaid modulating potentials whereby said auxiliary tube feeds energy tosaid load circuit on the upward swings of said modulation potential anadditional tube having electrodes connected with said direct-currentsource and having its impedance reduced by said modulating potentials onthe downward swings of said modulating potentials to increase thecurrent drawn by said additional tube from said direct current sourceand thereby reduce the output of said main tube on the downward swingsof said modulating potentials. j

12. In a modulating system, a main amplifier tube having electrodes, asource of high frequency voltage coupled to electrodes of said tube tobe amplified by aid tube, means for applying operating potentials tosaid electrodes, a load circuit for said main tube, a source ofmodulating voltage, an auxiliary amplifier'tube coupled to both of saidsources, means for biasing said auxiliary amplifier tube highly negativesothat said auxiliary amplifier tube amplifies high frequency voltagederived from said source only during substantially one half of eachcycle of said modulating voltage, means coupling the output of saidauxiliary amplifier tube to said load circuit, a modulator tube havingelectrodes connected with the electrodes of said main tube and to themeans for applying operating potentials to the electrodes of said maintube, and means for reducing the impedance between the electrodes ofsaid modulator tube during substantially the other half of each cycle ofmodulating voltage to thereby divert current supplied by said means forapplying operating potentials to the electrodes of said main tube fromsaid main tube and thus lower said main tubes output during said othersubstantially half cycle.

JAMES TWA'I'T.

